The present invention generally pertains to cable structures and, more particularly, to an improved spliceless stepped tow line especially adapted for towing objects and a method for fabricating the same. It is customary practice to tow target objects behind aircraft through the use of wire tow lines and the like. However, the modern target objects must be towed at several thousand yards behind its aircraft and at supersonic speeds to provide a practical and safe simulation of an actual target for either ground-to-air or air-to-air missles. One particular reason for the fact that the tow lines must be of sufficient length in that many missles are of the heat-seeking variety and, therefore, the towing aircraft must be kept well outside their range or otherwise it may be subject to damage. An important consideration in the construction and fabrication of these tow lines is that the specific tow cable or line have a relatively high strength to diameter ratio. The purpose for such high strength to diameter ratio is to overcome the detrimental effects produced by wind drag especially at the contemplated speeds and tow lengths that are currently used.
One prior art attempt to provide a tow line which adequately responds to the needs required for tow lines in modern high speed applications is generally described in U.S. Pat. No. 3,234,722. In this particular patent, the various layers forming a tow cable were radially compacted so as to effect an appreciable reduction in the diameter of a cable of a given rated strength to thereby correspondingly decrease the wind drag factor which would act upon such tow line.
Another prior art approach has been to utilize stepped tow lines for high speed applications, such as of the type generally described in U.S. Pat. No. 3,120,734. Since, as aforementioned, the general type of tow line utilized for towing aircraft targets extends several thousand feet the weight of such line becomes an important factor in determining the overall effectiveness thereof. Additionally, in recent years the wire lines have been required in which the material of the line is stressed to rather near its ultimate strength and at the same time such line must be capable of passing over sheaves and around a drum or reel. By reason of the relatively high speed of today's aircraft and the requirement for lengthy tow lines, a uniform diameter tow lines will tend to part under tension generated by its own air resistance or drag even though the weight in drag of the towed objects is comparatively small. Under such circumstances it is essential that the outer end of the line be as small as possible so as to minimize the drag as the diameter increases towards the inner end to compensate for tension generated by the drag of the line outboard. Theoretically, at least, the line should be generally tapered. Although the latter aforementioned patent discloses a technique for producing stepped tapered tow lines, such technique requires that the tow line be spliced together so as to join strands of different diameters. This form of stepped tapered tow line has been generally successful in many applications.
With, however, the advent of larger scale size targets, such as in the order to two-thirds the actual size, there is a corresponding resultant increase in the towing load such that there is an increase in tension generated by the drag or air resistance. Accordingly, to compensate for such resulting increases in drag, it is desirable to have the outer end of the line as small as possible as well as have the general strength of the tow line increased.
Unfortunately, spliced stepped tow lines of the type generally described in the above referenced patent whenever employed in typical types of turbine driven payout and retraction devices currently adapted for use on operational aircraft are limited to a particular size or diameter of tow line. This is by virtue of the fact that the overlie strand and flat wire armour at each of the joints tend to limit the diameter of the top sized strand in the tow line. Such diameters which may be used, however, are generally not as strong and, therefore, tend to be inadequate to meet the demands placed thereon. As noted, these increased demands are attributable to the fact that in modern type tow targets there is a general increase in the size and weight of the towed target object, as well as an increased length of the tow line necessitated by the use of extended scopes for the target practice.
Present day approaches to overcome the unsatisfactory results attendant with the use of the known spliced stepped tow lines under these circumstances are the utilization of constant diameter tow lines having greater diameter and greater number of individual wires stranded together so as to provide the adequate strength necessary for the purposes aforementioned. By way of example, these present day approaches have utilized 0.180 3 .times. 7 and 0.230 3 .times. 19 constant diameter wires which respectively have reel capacities of approximately 10,000 and 8,000 feet. Such constant diameter lines, of course, are not as suitable as would be desired, especially in light of the stress and weight imposed thereby.